Optimizing sensitivity of optical metrology measurements

ABSTRACT

Provided is a method of optimizing sensitivity of measurements of an optical metrology tool using two or more illumination beams directed to a structure on a workpiece comprising selecting target structures for measurement, obtaining diffraction signals off the selected structures as a function of angle of incidence for each illumination beam, determining a selected angle of incidence for each of the two or more illumination beams, setting sensitivity objectives for optical metrology measurements, developing a design for the optical metrology tool to achieve the corresponding selected angle of incidence of the two or more illumination beams, obtaining sensitivity data using the optical metrology tool, and if the sensitivity objectives are not met, adjusting the selection of target structures, the selected angle of incidence of the two or more illumination beams, the sensitivity objectives, and/or the design of the optical metrology tool, and iterating the developing of the design, obtaining sensitivity data, and comparing sensitivity data to sensitivity objectives until the sensitivity objectives are met.

BACKGROUND

1. Field of Invention

The present application generally relates to the design of an opticalmetrology tool to measure a structure formed on a workpiece, and, moreparticularly, to a method and an apparatus for controlling angles ofincidence (AOI) of multiple illumination beams in an objective lensassembly and a method of optimizing optical metrology measurementsensitivity.

2. Related Art

Optical metrology involves directing an incident beam at a structure ona workpiece, measuring the resulting diffraction signal, and analyzingthe measured diffraction signal to determine various characteristics ofthe structure. The workpiece can be a wafer, a substrate, photomask or amagnetic medium. In manufacturing of the workpieces, periodic gratingsare typically used for quality assurance. For example, one typical useof periodic gratings includes fabricating a periodic grating inproximity to the operating structure of a semiconductor chip. Theperiodic grating is then illuminated with an electromagnetic radiation.The electromagnetic radiation that deflects off of the periodic gratingare collected as a diffraction signal. The diffraction signal is thenanalyzed to determine whether the periodic grating, and by extensionwhether the operating structure of the semiconductor chip, has beenfabricated according to specifications.

In one conventional system, the diffraction signal collected fromilluminating the periodic grating (the measured diffraction signal) iscompared to a library of simulated diffraction signals. Each simulateddiffraction signal in the library is associated with a hypotheticalprofile. When a match is made between the measured diffraction signaland one of the simulated diffraction signals in the library, thehypothetical profile associated with the simulated diffraction signal ispresumed to represent the actual profile of the periodic grating. Thehypothetical profiles, which are used to generate the simulateddiffraction signals, are generated based on a profile model thatcharacterizes the structure to be examined. Thus, in order to accuratelydetermine the profile of the structure using optical metrology, aprofile model that accurately characterizes the structure should beused.

With increased requirement for throughput, decreasing size of the teststructures, smaller spot sizes, and lower cost of ownership, there isgreater need to optimize design of optical metrology systems to meetseveral design goals. Characteristics of the optical metrology systemincluding throughput, range of measurement capabilities, accuracy andrepeatability of diffraction signal measurements are essential tomeeting the increased requirement for smaller spot size and lower costof ownership of the optical metrology system. Selection of number ofillumination beams, light sources, angle of incidence, and optimizationof optical measurement sensitivity contribute to the above objectives.

SUMMARY

Provided is a method of optimizing sensitivity of measurements of anoptical metrology tool using two or more illumination beams directed toa structure on a workpiece comprising selecting target structures formeasurement, obtaining diffraction signals off the selected structuresas a function of angle of incidence for each illumination beam,determining a selected angle of incidence for each of the two or moreillumination beams, setting sensitivity objectives for optical metrologymeasurements, developing a design for the optical metrology tool toachieve the corresponding selected angle of incidence of the two or moreillumination beams, obtaining sensitivity data using the opticalmetrology tool, and if the sensitivity objectives are not met, adjustingthe selection of target structures, the selected angle of incidence ofthe two or more illumination beams, the sensitivity objectives, and/orthe design of the optical metrology tool, and iterating the developingof the design, obtaining sensitivity data, and comparing sensitivitydata to sensitivity objectives until the sensitivity objectives are met.

DETAILED DESCRIPTION Brief Description of Drawings

FIG. 1 is an architectural diagram illustrating an exemplary embodimentwhere an optical metrology system can be utilized to determine theprofiles of structures formed on a semiconductor wafer.

FIG. 2 depicts an exemplary optical metrology system in accordance withembodiments of the invention.

FIG. 3 depicts an exemplary architectural diagram depicting prior artobjective lens assembly using a single convex secondary mirror.

FIG. 4 depicts an architectural diagram depicting an objective lensassembly using separate illumination and detection convex secondarymirrors for two illumination beams.

FIG. 5A depicts an exemplary top-view of an architectural diagram of anobjective lens assembly using separate illumination and detection convexsecondary mirrors for two illumination beams whereas FIG. 5B depicts afacet mirror used to help combine illumination beams from two sourcesand a similar facet mirror used to help separate two detection beams.FIG. 5C depicts a knife-edge mirror used to help separate two detectionbeams, and a similar knife-edge mirror used to help combine twoillumination beams.

FIG. 6A depicts an exemplary flowchart of a method of determiningprofile parameters of a structure with an objective lens assembly usingseparate illumination and detection convex secondary mirrors for twoillumination beams whereas FIG. 6B depicts an exemplary flowchart of amethod of determining profile parameters of a structure with anobjective lens assembly using separate illumination and detection convexsecondary mirrors for three or more illumination beams.

FIG. 7 depicts an exemplary flowchart for optimizing the design of ametrology tool using measurement sensitivity objectives.

FIG. 8 depicts exemplary graph of optical metrology measurementsensitivity as a function of angle of incidence of the illuminationbeam.

FIG. 9 depicts an exemplary prior art block diagram of a system fordetermining and utilizing profile parameters for automated process andequipment control.

FIG. 10 depicts an exemplary prior art flowchart for optical metrologymeasurements of a structure on the workpiece, extracting structureprofile parameters and controlling a fabrication process.

DETAILED DESCRIPTION

In order to facilitate the description of the present invention, asemiconductor wafer may be utilized to illustrate an application of theconcept. The systems and processes equally apply to other workpiecesthat have reflective surfaces. The workpiece may be a wafer, asubstrate, photomask, magnetic medium or the like. Furthermore, in thisapplication, the term structure when it is not qualified refers to apatterned structure. In the following description, for purposes ofexplanation and not limitation, specific details are set forth, such asa particular geometry or layout of an optical metrology system,descriptions of various components and methods used therein. Referencethroughout this specification to “one embodiment” or “an embodiment”means that a particular feature, structure, material, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the invention, but do not denote that they are present inevery embodiment. Thus, the appearances of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily referring to the same embodiment ofthe invention. Furthermore, the particular features, structures,materials, or characteristics may be combined in any suitable manner inone or more embodiments. It should be understood that the invention maybe practiced in other embodiments that depart from these specificdetails. Referring now to the drawings, like reference numeralsdesignate identical or corresponding parts throughout the several views.

FIG. 1 is an architectural diagram illustrating an exemplary embodimentwhere optical metrology can be utilized to determine the profiles orshapes of structures fabricated on a semiconductor wafer. The opticalmetrology system 40 includes a metrology beam source 41 projecting ametrology illumination beam 43 at the target structure 59 of a wafer 47.The metrology beam 43 is projected at an incidence angle θ towards thetarget structure 59. The diffracted detection beam 49 is measured by ametrology beam receiver 51. A measured diffraction signal 57 istransmitted to a processor 53. The processor 53 compares the measureddiffraction signal 57 against a simulator 60 of simulated diffractionsignals and associated hypothetical profiles representing varyingcombinations of critical dimensions of the target structure andresolution. The simulator can be either a library that consists of amachine learning system, pre-generated data base and the like (this islibrary system), or on demand diffraction signal generator that solvesthe Maxwell equation for a giving profile (this is regression system).In one exemplary embodiment, the diffraction signal generated by thesimulator 60 instance best matching the measured diffraction signal 57is selected. The selected hypothetical profile and associated criticaldimensions of the selected simulator 60 instance are assumed tocorrespond to the actual cross-sectional shape and critical dimensionsof the features of the target structure 59. The optical metrology system40 may utilize a reflectometer, an ellipsometer, or other opticalmetrology device to measure the diffraction beam or signal. An opticalmetrology system is described in U.S. Pat. No. 6,943,900, entitledGENERATION OF A LIBRARY OF PERIODIC GRATING DIFFRACTION SIGNAL, issuedon Sep. 13, 2005, which is incorporated herein by reference in itsentirety.

Simulated diffraction signals can be generated by applying Maxwell'sequations and using a numerical analysis technique to solve Maxwell'sequations. It should be noted that various numerical analysistechniques, including variations of RCWA, can be used. For a more detaildescription of RCWA, see U.S. Pat. No. 6,891,626, titled CACHING OFINTRA-LAYER CALCULATIONS FOR RAPID RIGOROUS COUPLED-WAVE ANALYSES, filedon Jan. 25, 2001, issued May 10, 2005, which is incorporated herein byreference in its entirety.

Simulated diffraction signals can also be generated using a machinelearning system (MLS). Prior to generating the simulated diffractionsignals, the MLS is trained using known input and output data. In oneexemplary embodiment, simulated diffraction signals can be generatedusing an MLS employing a machine learning algorithm, such asback-propagation, radial basis function, support vector, kernelregression, and the like. For a more detailed description of machinelearning systems and algorithms, see U.S. patent application Ser. No.10/608,300, titled OPTICAL METROLOGY OF STRUCTURES FORMED ONSEMICONDUCTOR WAFERS USING MACHINE LEARNING SYSTEMS, filed on Jun. 27,2003, which is incorporated herein by reference in its entirety.

FIG. 2 shows an exemplary block diagram of an optical metrology systemin accordance with embodiments of the invention. In the illustratedembodiment, an optical metrology system 100 can comprise a lampsubsystem 105, and at least two optical outputs 106 from the lampsubsystem 105 can be transmitted to an illuminator subsystem 110. Thelamp subsystem 105 may include a first lamp, for example, a deuteriumlamp generating an illumination beam with a plurality of wavelengths.The wavelength range may be 180 to 400 nm or 180 to 350 nm. Otherwavelength ranges can be used. The lamp subsystem 105 may also include asecond lamp, for example, a xenon lamp generating an illumination beamwith a plurality of wavelengths. The wavelength range may be 200 to 900nm or 250 to 900 nm. Other wavelength ranges can be used. Alternatively,lamp subsystem 105 may include other light sources or combinations oflight sources and other combinations of wavelength ranges.

At least two optical outputs 111 from the illuminator subsystem 110 canbe transmitted to a selector subsystem 115. The selector subsystem 115can send at least two signals 116 to a beam generator subsystem 120. Inaddition, a reference beam 126 is split from the main beam 116 anddirected to subsystem 125 that can be used to provide reference outputs.A second calibration subsystem 127 provides a wavelength calibrationlamp that can be used as a source to generate calibration beam 128, thesource selected between the illuminator subsystem 110 and calibrationsubsystem 127, for example, with a flip-in mirror (not shown). The wafer101 is positioned using an X-Y-Z-theta stage 102 where the wafer 101 isadjacent to a wafer alignment sensor 104, supported by a platform base103.

The optical metrology system 100 can comprise a polarizer subsystem 129and a first selectable reflection subsystem 130 that can be used todirect at least two outputs 121 from the polarizer subsystem 129 on afirst path 131 when operating in a first mode “LOW AOI” or on a secondpath 132 when operating in a second mode “HIGH AOI”. When the firstselectable reflection subsystem 130 is operating in the first mode “LOWAOI”, at least two of the outputs 121 from the polarizer subsystem 129can be directed to a first reflection subsystem 140 as outputs 131, andat least two outputs 141 from the first reflection subsystem can bedirected to a low angle focusing subsystem 145, When the firstselectable reflection subsystem 130 is operating in the second mode“HIGH AOI”, at least two of the outputs 121 from the polarizer subsystem129 can be directed to a high angle focusing subsystem 135 as outputs132. Alternatively, other modes in addition to “LOW AOI” and “HIGH AOI”may be used and other configurations may be used.

When the metrology system 100 is operating in the first mode “LOW AOI”,at least two of the outputs 146 from the low angle focusing subsystem145 can be directed to the wafer 101. For example, a low angle ofincidence can be used. When the metrology system 100 is operating in thesecond mode “HIGH AOI”, at least two of the outputs 136 from the highangle focusing subsystem 135 can be directed to the wafer 101. Forexample, a high angle of incidence can be used. Alternatively, othermodes may be used and other configurations may be used.

The optical metrology system 100 can comprise a low angle collectionsubsystem 155, a high angle collection subsystem 165 a second reflectionsubsystem 150, and a second selectable reflection subsystem 160.

When the metrology system 100 is operating in the first mode “LOW AOI”,at least two of the outputs 156 from the wafer 101 can be directed tothe low angle collection subsystem 155. For example, a low angle ofincidence can be used. In addition, the low angle collection subsystem155 can process the outputs 156 obtained from the wafer 101 and lowangle collection subsystem 155 can provide outputs 151 to the secondreflection subsystem 150, and the second reflection subsystem 150 canprovide outputs 152 to the second selectable reflection subsystem 160.When the second selectable reflection subsystem 160 is operating in thefirst mode “LOW AOI” the outputs 152 from the second reflectionsubsystem 150 can be directed to the analyzer subsystem 170. Forexample, at blocking elements can be moved allowing the outputs 152 fromthe second reflection subsystem 150 to pass through the secondselectable reflection subsystem 160 with a minimum amount of loss.

When the metrology system 100 is operating in the second mode “HIGHAOI”, at least two of the outputs 166 from the wafer 101 can be directedto the high angle collection subsystem 165. For example, a high angle ofincidence can be used. In addition, the high angle collection subsystem165 can process the outputs 166 obtained from the wafer 101 and highangle collection subsystem 165 can provide outputs 161 to the secondselectable reflection subsystem 160. When the second selectablereflection subsystem 160 is operating in the second mode “HIGH AOI” theoutputs 162 from the second selectable reflection subsystem 160 can bedirected to the analyzer subsystem 170.

When the metrology system 100 is operating in the first mode “LOW AOI”,low incident angle, the output beam 162 is directed to the analyzersubsystem 170, and when the metrology system 100 is operating in thesecond mode “HIGH AOI”, high incident angle data from the wafer 101,output beam 162, generated from output beam 161, is directed to theanalyzer subsystem 170.

Metrology system 100 can include at least two measurement subsystems175. At least two of the measurement subsystems 175 can include at leasttwo detectors such as spectrometers. For example, the spectrometers canoperate from the Deep-Ultra-Violet to the visible regions of thespectrum.

The metrology system 100 can include a camera subsystems 180,illumination and imaging subsystems 182 coupled to the camera subsystems180. In some embodiments, the metrology system 100 can includeauto-focusing subsystems 190. Alternatively, other focusing techniquesmay be used.

One or more of the controllers (not shown) in at least one of thesubsystems (105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160,165, 170, 175, 180, 182, 190, and 195) can be used when performingmeasurements of the structures. A controller can receive real-signaldata to update subsystem, processing element, process, recipe, profile,image, pattern, and/or model data. One or more of the subsystems (105,110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175,180, 182, and 190) can exchange data using at least two SemiconductorEquipment Communications Standard (SECS) messages, can read and/orremove information, can feed forward, and/or can feedback theinformation, and/or can send information as a SECS message.

Those skilled in the art will recognize that one or more of thesubsystems (105, 110, 115,120, 125, 130, 135, 140, 145, 150, 155, 160,165, 170, 175, 180, 182, 190, and 195) can include computers and memorycomponents (not shown) as required. For example, the memory components(not shown) can be used for storing information and instructions to beexecuted by computers (not shown) and may be used for storing temporaryvariables or other intermediate information during the execution ofinstructions by the various computers/processors in the metrology system100. One or more of the subsystems (105, 110, 115, 120, 125, 130, 135,140, 145, 150, 155, 160, 165, 170, 175, 180, 185, and 190) can includethe means for reading data and/or instructions from a computer readablemedium and can comprise -the means for writing data and/or instructionsto a computer readable medium. The metrology system 100 can perform aportion of or all of the processing steps of the invention in responseto the computers/processors in the processing system executing at leasttwo sequences of at least two instructions contained in a memory and/orreceived in a message. Such instructions may be received from anothercomputer, a computer readable medium, or a network connection. Inaddition, one or more of the subsystems (105, 110, 115, 120, 125, 130,135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 182, and 190) cancomprise control applications, Graphical User Interface (GUT)components, and/or database components.

It should be noted that the beam when the metrology system 100 isoperating in the first mode “LOW AOI” with a low incident angle datafrom the wafer 101 all the way to the measurement subsystems 175,(output 156, 151, 152, 162, and 171) and when the metrology system 100is operating in the second mode “HIGH AOI” with a high incident angledata from the wafer 101 all the way to the measurement subsystems 175,(output 166, 161, 162, and 171) is referred to as diffraction signal(s).

FIG. 3 is an architectural diagram depicting prior art objective lensassembly using a single convex secondary mirror for the illumination anddetection. The prior art objective lens assembly 300 comprise anillumination source (not shown) that generates a broadband illuminationbeam 310, the illumination beam 310 passing through an illuminationaperture 305 and projected to the secondary convex mirror 320. Theillumination beam 310 is reflected by the secondary convex mirror 320 asbeam 315 onto a primary illumination mirror 325 and is reflected asillumination beam 330 onto a structure (not shown) on the workpiece 335.The illumination beam 330 is diffracted by the structure on theworkpiece 335 as detection beam 340 onto the detection primary mirror345, reflected as detection beam 350 onto the secondary convex mirror320 and reflected as detection beam 355, passing through a detectionaperture 360 and onto a detector (not shown). Mirrors 325 and 345 areusually combined into one larger spherical mirror. Objective lensassemblies in prior art may include a second illumination beam directedthrough similar optical components as shown in FIG. 3. A workpiece mayinclude a wafer, substrate, or photomask. The use of a single convexsecondary mirror used for illumination and detection does not providethe flexibility of achieving or getting close to an optimum angle ofincidence when the numerical aperture of the beam 330 is fixed.

In addition to having an optimum illumination angle of incidence,optical metrology systems will also have an optimum illuminationnumerical aperture (NA). A larger NA will have the desirable effects ofincreasing signal strength and decreasing the diffraction-limited spotsize on the workpiece, but it will also have the undesired effect ofdistorting the spectral signal. The NA is thus chosen to be sufficientto produce the required spot size and signal strength, but not too largethat it produces a spectral distortion that can not be modeledaccurately with reasonable computation effort. Given a particularrequired NA of beam 330 and other objective design constraints such asthe focal length, magnification, working distance, and sensitivity totolerances, there is a minimum practical value for θ. When the primaryand secondary are positioned relative to each other to achieve all otherconstraints, the edge of the secondary mirror limits the minimum angleθ. This limit can often prevent use of the optimum value for θ.

FIG. 4 is an architectural diagram 400 depicting an optical metrologytool including an objective lens assembly using a separate convexsecondary mirror 430 for the illumination beams and another separateconvex secondary mirror 474 for the detection beams, in an exemplaryembodiment using two illumination beams. The optical metrology tool 400comprises light sources (not shown), a set of beam separation optics 402for spatially separating the illumination and detection beams, anillumination polarizer 416, a detection polarizer 496, an objective lensassembly 404 and a motion control system 406 comprising a tilting device442 and a stage 444. The set of beam separation optics 402 comprises aknife-edge mirror 412, an illumination faceted mirror 418, a detectionfaceted mirror 490, and a detection knife-edged mirror 498. The firstillumination source (not shown) generates a first illumination beam 410directed to the illumination polarizer 416 and onto illumination facetedmirror 418. The second illumination source (not shown) generates asecond illumination beam 414, reflected by knife-edge illuminationmirror 412, on through the illumination polarizer 416 onto theillumination faceted mirror 418. The first illumination source may be adeuterium light source generating a first illumination beam 410 with awavelength range of about 180 to 400 nm. The second illumination sourcemay be a xenon lamp generating a second illumination beam 414 with awavelength range of about 200 to 900 nanometers (nm). Alternatively, thefirst illumination beam 410 may have a wavelength range of 180 to 350 nmand the second illumination beam 414 may have a wavelength range of 300to 900 nm. Other illumination sources or combination of light sourcesmay be used depending on the desired range of wavelengths generated forthe set of illumination beams.

As mentioned above, referring to FIGS. 4, 5B, and 5C, the set of beamseparation optics 402 comprises a knife-edge mirror 412, an illuminationfaceted mirror 418, a detection faceted mirror 490, and a detectionknife-edged mirror 498. The illumination faceted mirror 418 of FIG. 4 isfurther depicted in FIG. 5B in an exemplary embodiment 400A where thefirst illumination beam 410 is reflected by a first facet 418A as firstillumination beam 424 and the second illumination beam 414 is reflectedby a second facet 418B as second illumination beam 426. Beams 410 and414 originate from two spatially-separated point-like sources. Afterreflecting from facets 418A and 418B, beams 424 and 426 appear tooriginate from a single point-like virtual source allowing the lightfrom both beams to be eventually focused onto one point on theworkpiece. However, beams 424 and 426 are now separated in angle so thatthey enter separate illumination apertures 420 and 422. Apertures 420and 422 serve to define the exact shape, NA and angle of incidence ofthe illumination beams 436 and 434. The angle, β, of the first facet418A and the second facet 418B can be in the range of 0.3 to 0.8degrees. Alternatively, the angle of the first facet 418A and the secondfacet 418B can be in the range of 0.01 to 45.0 degrees. Other facetangles can be used to achieve the spatial separation of the beamsdesired. A similar detection faceted mirror (490 in FIG. 4) is in thedetection path for spatially separating the detection beams (480 and482) reflected off the detection secondary mirror 474. The detectionknife-edged mirror 498 of FIG. 4 is further depicted in FIG. 5C in anexemplary embodiment 400B where the detection beams (494 and 492) arespatially separated and directed to spectroscopic detectors (not shown).The first detection beam 494 is allowed to go trough while the seconddetection beam 492 is reflected by detection knife-edge mirror 498 ontoa different spectroscopic detector (not shown). Another knife-edgemirror (412 of FIG. 4) is used to reflect the second illumination beam414 onto the illumination faceted mirror 418.

Referring to FIG. 4, the illumination secondary mirror 430 is disposedproximate but not connected to the detection secondary mirror 474. Thefirst illumination beam 410 is reflected from the illumination facetedmirror 418, through an illumination aperture 422 as first illuminationbeam 424 onto the illumination secondary mirror 430, reflected from theillumination secondary mirror 430 onto the illumination primary mirror432 and reflected onto the structure on the workpiece 440 at a firstangle of incidence θ₁. The second illumination beam 414 is reflectedfrom the illumination faceted mirror 418 through an illuminationaperture 420 as second illumination beam 426 onto the illuminationsecondary mirror 430, reflected from the illumination secondary mirror430 onto the illumination primary mirror 432 and reflected onto thestructure (not shown) on the workpiece 440 at a second angle ofincidence θ₂. The illumination secondary mirrors 430 and the detectionsecondary mirror 474 are convex mirrors whereas the primary illuminationprimary mirror 432 and the detection primary mirror 460 are concavemirrors. In one embodiment, the first angle of incidence, θ₁, can besubstantially 30 degrees and the second angle of incidence θ₂, can besubstantially 18 degrees. Alternatively, θ₁ can be within a range of 25to 35 degrees and θ₂ can be within a range of 10 to 22 degrees.Alternatively, θ₁ can be within a range of 40 to 65 degrees and θ₂ canbe within a range of 8 to 25 degrees. Other ranges of angles ofincidence can also be used.

As mentioned above, use of a separate illumination secondary mirror 430and a separate detection secondary mirror 474 provide flexibility inachieving desired angles of incidence of the illumination beams onto thestructure by adjusting the position and rotation of the secondarymirrors, the primary mirrors, or both relative to the structure. Theflexibility of moving the primary mirrors relative to the secondarymirrors to achieve a desire angle of incidence is further enhanced bythe absence of an unused portion of a single secondary mirror (321 ofFIG. 3). By eliminating mirror section 321 in FIG. 3, the illuminationprimary and secondary mirrors can be rotated together as one unit aboutthe focus point on the work piece to reduce the angle of incidence.Likewise the same can be done on the collection side to create anobjective like the objective lens assembly 404 in FIG. 4. In anotherembodiment, in order to compensate for the angle of incidence tolerancein manufacturing, a tilt device 442 is capable of matching the tilt ofthe workpiece 440 to the tilt of the objective lens assembly 404.Furthermore, in another embodiment, the illumination apertures, (420 and422), are slightly larger than the detection apertures (488 and 486) inpart to compensate for the angle of incidence tolerance inmanufacturing. The separate secondary mirrors can be utilized to makethe angles of incidence as close as possible to the calculated optimumangle of incidence based on numerical aperture of a given light source,such as a deuterium lamp, a xenon lamp, and the kind. Numerical aperture(NA) is determined by taking the sine of the angle of the cone of lightgenerated by the light source. In one embodiment, the NA of the firstand second light source is substantially 0.07. Alternatively, the NA ofthe first and second light sources may be within a range of 0.05 to0.09. Alternatively, the NA of the first and second light sources may bewithin a range of 0.07 to 0.12.

Illumination beam 434 is diffracted by the structure on the workpiece440 as a detection beam 450, reflected by detection primary mirror 460as detection beam 472 onto the detection convex mirror 474, andreflected as detection beam 480 through the detection aperture 488.Illumination beam 436 is diffracted by the structure on the workpiece440 as a detection beam 452 and reflected by detection primary mirror460 as detection beam 470 onto detection secondary convex mirror 474,reflected as detection beam 482 through the detection aperture 486. Bothdetection beams passing through the detection apertures (486 and 488)onto detection faceted mirror 490, go through detection polarizer 496.As mentioned above, the first detection beam 494 is projected onto afirst spectroscopic detector (not shown) and the second detection beam492 is reflected by detection knife-edged mirror 498 onto a secondspectroscopic detector (not shown) where the diffraction signals aremeasured using the first and second spectroscopic detectors.

FIG. 5A depicts a top-view 500 of an architectural diagram of anobjective lens assembly using a separate illumination convex secondarymirror for two illumination beams and a separate detection convexsecondary mirror such as the one depicted in FIG. 4. With reference toFIG. 5A, the top view 500 of an architectural diagram of an exemplaryembodiment utilizing two illumination beams (not shown) comprises aprimary illumination mirror 508, a separate illumination secondarymirror 512, and two illumination apertures (532 and 536) of theobjective lens assembly 504. The structure (not shown) on the workpiece524 may or may not be in the center of the objective lens assembly 500depending on selected footprint and design of the metrology tool. Thedetection side comprises a primary detection mirror 540, separatedetection secondary mirror 528 and two detection apertures (516 and 520)of the objective lens assembly 504.

FIG. 6A depicts an exemplary flowchart of a method of determiningprofile parameters of a structure using an objective lens assembly withtwo illumination beams whereas FIG. 6B depicts an exemplary flowchart ofa method of determining profile parameters of a structure using anobjective lens assembly with three or more illumination beams. Referringto FIG. 6A, in step 700, two illumination beams are generated, each beamhaving a plurality of wavelengths. In one embodiment, a firstillumination beam can be generated using a deuterium light source,generating a beam in the range of 180 to 400 nm. A second illuminationbeam can be generated using a xenon lamp and generates a beam in therange of 200 to 900 nm. Alternatively, the first illumination beam maybe in the range of 180 to 380 nm and the second illumination beam may bein the range 180 to 900 nm. Other ranges of wavelengths using otherlight sources can also be utilized. In step 704, the two illuminationbeams are projected through a set of beam separation optics, apolarizer, and a corresponding illumination aperture onto anillumination secondary mirror. In one embodiment, the configuration ofthe illumination secondary mirror is as described for the illuminationsecondary mirror 430 in FIG. 4. Other configurations of two illuminationsecondary mirrors can also be used. In step 708, the two illuminationbeams are reflected onto an illumination primary mirror.

In one embodiment, the illumination primary mirror is a single concavemirror configured such that in combination with the separateillumination secondary mirror, the angles of incidence of theillumination beams onto the structure on the workpiece can be adjustedto be close or equal to a set or calculated optimum angle of incidencebased on the numerical aperture of the light sources. In one embodiment,as depicted in FIG. 4, the first angle of incidence, θ₁, can besubstantially 30 degrees and the second angle of incidence, θ₂, can besubstantially 18 degrees. Alternatively, θ₁ can be within a range of 25to 35 degrees and θ₂ can be within a range of 10 to 22 degrees.Alternatively, θ₁ can be within a range of 40 to 65 degrees and θ₂ canbe within a range of 8 to 25 degrees. Other ranges of angles ofincidence can also be used. Still referring to FIG. 6A, the illuminationbeams reflected from the illumination primary mirror are projected ontothe structure on the workpiece at or close to optimum angles ofincidence, generating two detection beams, step 712.

In step 716, the two detection beams are reflected onto a separatedetection secondary mirror. In step 720, the two detection beams arereflected onto a detection primary mirror, pass through a correspondingdetection aperture and other optical components such as a set of beamseparation optics and a polarizer. In step 724, the two detection beamsare measured using one or more spectroscopic detectors, generating adiffraction signal. In step 728, the diffraction signal is used todetermine one or more profile parameters of the structure. For detailsof using a diffraction signal to determine a structure profileparameter, refer to U.S. Pat. No. 6,943,900, entitled GENERATION OF ALIBRARY OF PERIODIC GRATING DIFFRACTION SIGNAL, issued on Sep. 13, 2005,which is incorporated herein by reference in its entirety and to U.S.patent application Ser. No. 10/608,300, entitled OPTICAL METROLOGY OFSTRUCTURES FORMED ON SEMICONDUCTOR WAFERS USING MACHINE LEARNINGSYSTEMS, filed on Jun. 27, 2003, which are incorporated herein byreference in their entirety.

FIG. 6B depicts an exemplary flowchart of a method of determiningprofile parameters of a structure using an objective lens assembly withthree or more illumination beams. In step 750, three or moreillumination beams are generated, each beam having a plurality ofwavelengths. In step 754, the three or more illumination beams areprojected through corresponding illumination apertures onto three ormore illumination secondary mirrors. In step 758, the three or moreillumination beams are reflected onto two or more illumination primarymirrors. In one embodiment, an illumination primary mirror is configuredsuch that in combination with the corresponding illumination secondarymirror, the angle of incidence of each illumination beam onto thestructure on the workpiece can be set to be close or equal an optimumangle of incidence based on the numerical aperture of the correspondinglight source. The illumination beams reflected from the illuminationprimary mirrors are projected onto the structure on the workpiece at orclose to the optimum angles of incidence, generating three or moredetection beams, step 762. In step 766, the three or more detectionbeams are projected onto detection secondary mirrors. In step 770, thethree or more detection beams are reflected onto two or more detectionprimary mirrors, pass through corresponding detection apertures andother optical components such as beam separation optics and a polarizer.In step 774, the three or more detection beams are measured using two ormore spectroscopic detectors, generating a diffraction signal. In step778, the diffraction signal is used to determine one or more profileparameters of the structure.

FIG. 7 depicts an exemplary flowchart for optimizing the design of ametrology tool using measurement sensitivity objectives. In step 800,target structures are selected for measurement with an optical metrologytool using two or more illumination beams, each illumination beam havinga plurality of wavelengths. Target structures may include one or moretypes of one dimensional repeating structures on a wafer such asgratings, line and space structures, two dimensional repeatingstructures, and/or complex repeating structures comprising posts,contact holes, vias, islands, and concave or convex three dimensionalstructures, or combinations of two or more thereof. The opticalmetrology tool may be a reflectometer, ellipsometer, or hybrid opticalmetrology tools. In step 804, diffraction signals off the selectedstructures are obtained as a function of angles of incidence. Forexample, in a reflectometer, diffraction signals may be obtained toinclude reflectance intensity measurements as a function of angle ofincidence. In ellipsometers, diffraction signals would include intensityand change of polarization as a function of angle of incidence.Diffraction signals may be obtained using actual metrology tools,optical prototypes or numerical simulations. In step 808, an angle ofincidence is selected for each of the two or more illumination beams. Inone embodiment, for certain selected target structures, the inventorsselected to use two illumination beams, namely, one generated using axenon lamp and another using a deuterium light source. The calculatedoptimum angle of incidence is substantially 18 degrees for the xenonlamp and substantially 30 degrees for the deuterium light source.Alternatively, the first angle of incidence, θ₁ of FIG. 4, can be withina range of 25 to 35 degrees and the second angle of incidence, θ₂ ofFIG. 4, can be within a range of 10 to 22 degrees. Alternatively, θ₁ canbe within a range of 40 to 65 degrees and θ₂ can be within a range of 8to 25 degrees. Other ranges of angles of incidence can also be used.

Selection of the angle of incidence is illustrated in a graph 850 inFIG. 8. The selected angle of incidence can be equal to or close to thecalculated optimum angle of incidence. Assume that target structures tobe measured with the metrology tool include a line and space gratingstructure, A, a repeating 2 dimensional structure including contactholes and vias, B, and a combination of isolated and dense structure C.Assume that the metrology tool is a non-normal reflectometer and usestwo illumination beams with the first illumination beam generated from axenon lamp. Also assume that the top critical dimension (CD) of thestructures is being measured. The reflectance of the three structures ismeasured using the reflectometer where critical dimension and the angleof incidence are varied. The changes in normalized reflectance perchange in CD width in nanometers are plotted as a function of angle ofincidence. Referring to the graph 850, the graphs for the structures A,B, and C depict the highest sensitivity at an angle of incidence ofabout 18 degrees, at point D. The selected angle of incidence for thefirst illumination beam in this case would be 18 degrees. Similar graphsor data can also be prepared for the second or subsequent illuminationbeams and can be used to determine the selected angle of incidence. Inanother embodiment, the metrology tool is an ellipsometer and change ofintensity and polarization are measured and used in the calculations.The selected angle of incidence for an illumination beam may also beverified by simulating the performance of a designed system andcomparing to design target specifications.

Referring to FIG. 7, in step 812, measurement sensitivity objectives areset for the selected structures. Based on the selected structures to bemeasured, certain profile parameters are considered critical dimensions(CDs). Examples of CDs are top width (TCD), mid width (MCD) and bottomwidth (BCD). Other profile parameters are often included, such asgrating heights, sidewall angle, and the like. Using the example above,assume that the TCD is critical for the selected structures, measurementsensitivity can be simulated with an algorithm such as RCWA as thereflectance change per 1 nm change of the TCD. The simulated sensitivitycan be used to verify if the design can meet the desired measurementrequirements, such as accuracy and precision. Sensitivity for one ormore different CDs may be calculated concurrently. In step 816, a designis developed for the optical metrology tool to achieve the selectedangles of incidence. In one embodiment, as depicted and described inconnection with FIG. 4, a separate illumination secondary mirror is usedin the illumination path and a separate detection secondary mirror isused in the detection path for an optical metrology tool with twoillumination beams. Other configurations using three or more beams canalso be used.

In step 820, sensitivity data using the optical metrology tool isobtained. Sensitivity data may be obtained using a variety of techniquesincluding using an assembled or manufactured version of the opticalmetrology tool, using an optical breadboard prototype that includes theessential components developed in the design in step 816, or usingsimulation of the diffraction signal using metrology modelingtechniques. Use of an optical breadboard prototype is discussed indetail in application Ser. No. 12/050,053 entitled METHOD OF DESIGNINGAN OPTICAL METROLOGY SYSTEM OPTIMIZED FOR OPERATING TIME BUDGET, filedon Mar. 17, 2008, which is incorporated herein by reference in itsentirety. In step 820, the sensitivity data obtained using the opticalmetrology tool is compared to the set sensitivity objectives. If the setsensitivity objectives are met, then one or more profile parameters ofthe structure is determined using a diffraction signal measured off thestructure using the optical metrology tool, step 832. The determined oneor more profile parameters is used to adjust at least one processparameter of the current fabrication process of the workpiece, or asubsequent process or a prior process, step 836.

Referring to step 824 of FIG. 7, if the set sensitivity objectives arenot met, the selection of target structures, the selected angles ofincidence, the sensitivity objectives, and/or the design of the opticalmetrology tool are adjusted, and developing of the optical metrologydesign, obtaining sensitivity data using the optical metrology tool, andcomparing sensitivity data obtained using the optical metrology toolcompared to the set sensitivity objectives are iterated until the setsensitivity objectives are met. Adjusting the selection of targetstructures may include identifying a structure that cannot be accuratelymeasured by the optical metrology tool and excluding this type ofstructure in the latter parts of the method. Adjusting the selectedangles of incidence may include changing the selected angle of incidencebased on new or more extensive diffraction signal data. The sensitivityobjectives may be raised or lowered based on the structure applicationor fabrication requirements. The design of the optical metrology toolmay be adjusted by selecting different light sources, changing thenumber of illumination beams, changing the set of wavelengths includedin an illumination beam, altering the design of the objective lensassembly, and/or using different optical components or a combination oftwo or more of the foregoing. As described in relation to FIGS. 4 and5A, the design of the objective lens assembly can be changed to useseparate illumination and detection secondary mirrors to provide theflexibility of achieving or getting close to an optimum angle ofincidence when the numerical aperture of the beam is fixed.

FIG. 9 is an exemplary prior art block diagram of a system fordetermining and utilizing profile parameters for automated process andequipment control. System 900 includes a first fabrication cluster 902and optical metrology system 904. System 900 also includes a secondfabrication cluster 906. Although the second fabrication cluster 906 isdepicted in FIG. 9 as being subsequent to first fabrication cluster 902,it should be recognized that second fabrication cluster 906 can belocated prior to first fabrication cluster 902 in system 900 (e.g. andin the manufacturing process flow).

A photolithographic process, such as exposing and/or developing aphotoresist layer applied to a wafer, can be performed using firstfabrication cluster 902. Optical metrology system 904 is similar tooptical metrology system 40 of FIG. 1. In one exemplary embodiment,optical metrology system 904 includes an optical metrology tool 908 andprocessor 910. Optical metrology tool 908 is configured to measure adiffraction signal off of the structure. The optical metrology tool 908can include an objective lens assembly as depicted in FIG. 4. Processor910 is configured to compare the measured diffraction signal measured bythe optical metrology tool designed to meet plurality of design goals toa simulated diffraction signal. As mentioned above, the simulateddiffraction is determined using a set of profile parameters of thestructure and numerical analysis based on the Maxwell equations ofelectromagnetic diffraction. In one exemplary embodiment, opticalmetrology system 904 can also include a library 912 with a plurality ofsimulated diffraction signals and a plurality of values of one or moreprofile parameters associated with the plurality of simulateddiffraction signals. As described above, the library can be generated inadvance; metrology processor 910 can compare a measured diffractionsignal off a structure to the plurality of simulated diffraction signalsin the library. When a matching simulated diffraction signal is found,the one or more values of the profile parameters associated with thematching simulated diffraction signal in the library is assumed to bethe one or more values of the profile parameters used in the waferapplication to fabricate the structure.

System 900 also includes a metrology processor 916. In one exemplaryembodiment, processor 910 can transmit the one or more values of the oneor more profile parameters to metrology processor 916. Metrologyprocessor 916 can then adjust one or more process parameters orequipment settings of the first fabrication cluster 902 based on the oneor more values of the one or more profile parameters determined usingoptical metrology system 904. Metrology processor 916 can also adjustone or more process parameters or equipment settings of the secondfabrication cluster 906 based on the one or more values of the one ormore profile parameters determined using optical metrology system 904.As noted above, second fabrication cluster 906 can process the waferbefore or after first fabrication cluster 902. In another exemplaryembodiment, processor 910 is configured to train machine learning system914 using the set of measured diffraction signals as inputs to machinelearning system 914 and profile parameters as the expected outputs ofmachine learning system 914.

FIG. 10 depicts an exemplary prior art flowchart for optical metrologymeasurements of a structure on the workpiece, extracting structureprofile parameters and controlling a fabrication process. In step 1000,one or more diffraction signals off a target structure on the workpieceare measured with an optical metrology system, where the metrologysystem includes a metrology tool with an objective lens assembly asdepicted in FIG. 4. In step 1004, at least one profile parameter of thestructure is determined using the measured diffraction signals. If theworkpiece is a semiconductor wafer, the one profile parameter may be atop critical dimension (CD), a bottom CD, or a sidewall angle. In step1008, at least one fabrication process parameter or equipment setting ismodified using the determined at least one profile parameter of thestructure. For example, if the workpiece is a wafer, the fabricationprocess parameter may include a temperature, exposure dose or focus,etchant concentration or gas flow rate. As mentioned above, the opticalmetrology system may be part of a standalone metrology module orintegrated in a fabrication cluster.

Although exemplary embodiments have been described, variousmodifications can be made without departing from the spirit and/or scopeof the present invention. For example, although an optical metrologytool with two illumination beams was primarily used to describe theembodiments of the invention; other configurations with three or moreillumination beams may also be used as mentioned above. For automatedprocess control, the fabrication clusters may be a track, etch,deposition, chemical-mechanical polishing, thermal, or cleaningfabrication cluster. Furthermore, the optical metrology tool designedusing the methods and apparatus of the invention are substantially thesame regardless of whether the optical metrology tool is integrated in afabrication cluster or used in a standalone metrology setup. Therefore,the present invention should not be construed as being limited to thespecific forms shown in the drawings and described above.

1. A method of optimizing sensitivity of measurements of an opticalmetrology tool using two illumination beams directed to a structure on aworkpiece, the structure having profile parameters, the methodcomprising: (a) selecting target structures for measurement using theoptical metrology tool; (b) obtaining diffraction signals off theselected structures as a function of angle of incidence for each of twoillumination beams, the two illumination beams having a plurality ofwavelengths; (c) determining a selected angle of incidence for each ofthe two illumination beams based on the obtained diffraction signals;(d) setting sensitivity objectives for optical metrology measurements;(e) developing a design for the optical metrology tool to achieve theselected angle of incidence of each of the two illumination beams; (f)obtaining sensitivity data using the optical metrology tool; (g) if thesensitivity objectives are not met, adjusting the selection of targetstructures, the selected angle of incidence of each of the twoillumination beams, the sensitivity objectives, and/or the design of theoptical metrology tool, and iterating (e), (f) and (g) until thesensitivity objectives are met; wherein the optical metrology toolcomprises an objective lens assembly, the objective lens assemblyincluding an illumination primary mirror, a detection primary mirror,and separate illumination and detection secondary mirrors.
 2. The methodof claim 1 wherein the workpiece is a wafer, a substrate, or aphotomask.
 3. The method of claim 2 wherein the structures includegratings, line and space structures, one or more types of onedimensional repeating structures, two dimensional repeating structures,and/or complex repeating structures comprising posts, contact holes,vias, islands, and concave or convex three dimensional structures, orcombinations of two thereof.
 4. The method of claim 1 wherein theobtained diffraction signals include measurements performed using areflectometer, ellipsometer or hybrid optical metrology tools.
 5. Themethod of claim 1 wherein the obtained diffraction signals are generatedusing hypothetical structures and simulated diffraction signals usingnumerical calculation techniques as a function of the angle of incidenceof the two illumination beams.
 6. The method of claim 1 wherein theselected angle of incidence of each of the two illumination beams isdetermined using statistical mathematical algorithms or software.
 7. Themethod of claim 1 wherein the sensitivity objectives include change ofdiffraction signal per unit change of one or more profile parameters. 8.The method of claim 7 wherein the sensitivity objectives include changeof reflectance intensity per unit change of one or more criticaldimension of the structure.
 9. The method of claim 7 wherein thesensitivity objectives include change of intensity and polarization perunit change of one or more profile parameters of the structure.
 10. Themethod claim 1 wherein adjusting the design of the optical metrologytool includes designing an objective lens assembly to project onto thestructure each of the two illumination beams substantially at thecorresponding angle of incidence of the selected angles of incidence.11. The method of claim 10 wherein the illumination secondary mirror andthe illumination primary mirror are positioned so as to make the angleof incidence substantially equal or close to the corresponding angle ofincidence of the selected angles of incidence.
 12. The method of claim 1wherein changing the design of the optical metrology tool includeschanging one or more light sources of the two illumination beams. 13.The method of claim 1 wherein changing the design of the opticalmetrology tool includes altering wavelengths included in the twoillumination beams.
 14. The method of claim 1 wherein changing thedesign of the optical metrology tool includes spatially separating andreflecting the two illumination beams, each illumination beam of the twoillumination beams using a illumination knife-edged mirror and aillumination faceted-mirror.
 15. The method of claim 1 wherein obtainingsensitivity data is performed using simulated diffraction signalsgenerated with a hypothetical set of profile parameters of the structureor using a prototype of the optical metrology tool.
 16. The method ofclaim 1 wherein adjusting the sensitivity objectives include changingthe measured diffraction signal intensity per unit change of the one ormore profile parameters.
 17. The method of claim 1 further comprisingusing the optical metrology tool to determine one or more profileparameters of the structure, the optical metrology tool integrated in afabrication cluster.
 18. A method of optimizing sensitivity ofmeasurements of an optical metrology tool using two illumination beamsdirected to a structure on a workpiece, the structure having profileparameters, the method comprising: (a) determining a selected firstangle of incidence for a first illumination beam generated from a xenonlamp and a selected second angle of incidence for a second illuminationbeam generated from a deuterium light source, the selected first andsecond angles of incidence based on the obtained diffraction signals offthe structure as a function of angle of incidence; (b) settingsensitivity objectives for optical metrology measurements; (c)developing a design for the optical metrology tool to achieve theselected angle of incidence of each of the two illumination beams; (d)obtaining sensitivity data using the optical metrology tool; (e) if thesensitivity objectives are not met, adjusting the selected angle ofincidence of each of the two illumination beams, the sensitivityobjectives, and/or the design of the optical metrology tool, anditerating (c), (d) and (e) until the sensitivity objectives are met. 19.The method of claim 18 wherein the determined one or more profileparameters of the structure is used to adjust at least one processparameter of a fabrication process in the fabrication cluster.
 20. Amethod of optimizing sensitivity of measurements of an optical metrologytool using three or more illumination beams directed to a structure on aworkpiece, the structure having profile parameters, the methodcomprising: (a) selecting target structures for measurement using theoptical metrology tool; (b) obtaining diffraction signals off theselected structures as a function of angle of incidence for each ofthree or more illumination beams, the three or more illumination beamshaving a plurality of wavelengths; (c) determining a selected angle ofincidence for each of the three or more illumination beams based on theobtained diffraction signals; (d) setting sensitivity objectives foroptical metrology measurements; (e) developing a design for the opticalmetrology tool to achieve the selected angle of incidence of each of thethree or more illumination beams; (f) obtaining sensitivity data usingthe optical metrology tool; (g) if the sensitivity objectives are notmet, adjusting the selection of target structures, the selected angle ofincidence of each of the three or more illumination beams, thesensitivity objectives, and/or the design of the optical metrology tool,and iterating (e), (f) and (g) until the sensitivity objectives are met;wherein the optical metrology toll comprises an objective lens assembly,the objective lens assembly including one or more illumination primarymirrors, one or more detection primary mirrors, and one or more pairs ofseparate illumination and detection secondary mirrors.